[PowerPC] [Clang] Define macros to identify quad-fp semantics

We have option -mabi=ieeelongdouble to set current long double to
IEEEquad semantics. Like what GCC does, we need to define
__LONG_DOUBLE_IEEE128__ macro in this case, and __LONG_DOUBLE_IBM128__
if using PPCDoubleDouble.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D90208